proto: add channel-lane bridge harness

[laynepenney]

Summary

• add a channel bridge prototype that derives #dev notifications from append-only lane events
• add stable event ids so watcher delivery can dedupe and resume cleanly
• extend the cross-mode stress harness to verify reconstructible lane timelines and watcher replay behavior
• document the watcher-first recommendation in the prototype README

Key design decision

• recommended delivery model is watcher, not synchronous posting during lane transitions
• rationale: lane transitions should stay durable and local even if channel delivery is down, and channel replay/dedupe is cleaner when the event log is the source of truth

Verification

• python3 -m py_compile gr2/prototypes/lane_workspace_prototype.py gr2/prototypes/channel_lane_bridge.py gr2/prototypes/cross_mode_lane_stress.py
• python3 gr2/prototypes/channel_lane_bridge.py recommend-delivery --json
• python3 gr2/prototypes/cross_mode_lane_stress.py --json

Notes

• this stays in the design/prototype/verify loop
• next remaining synapt integration slices are recall lane history surface, agent identity to unit mapping refinement, and org/policy to WorkspaceSpec compilation